A goal of neurobiology is to explain how neurons govern the amount of transmitter they release and thereby regulate synaptic transmission, we propose to use lobster (Homarus americanus) neuromuscular junction preparations to investigate the relationship between synaptic structure and transmitter release. The lobster provides a unique model system for these studies since it contains identifiable synapses which are suitable for ultrastructural studies and demonstrate a broad range of physiologically defined transmitter release characteristic. Long term changes in transmitter output associated with growth, development, and altered patterns of neuronal activity can also be measured at these synapses and related to changes in the number and distribution of synaptic contacts and the structural organization of the neurotransmitter release sites (active zones). I will examine an hypothesis which relates the structural organization of the axon terminals and the active zones they contain to differential control of transmitter release. I propose studies to examine the possibility that transmitter releases is controlled by four mechanisms: I) the density of large intramembrane particles at the active zones (active zone particles), 2) the number of synaptic vesicles poised to release transmitter at the active zones, and 4) The number and distribution of synaptic contacts. To examine these factors: 1) The number, size, and distribution of synaptic contacts (varicosities) will be determined in whole mounts in which axons have been stained with methylene blue, labeled with fluorescent lectins or antibodies, or injected with lucifer yellow; 2) the number and concentration of active zone particles will be measured in freezE-fracture views and correlated with high or low levels of transmitter release (high-output and low output synapses); 3) the number and position of synaptic vesicles will be determined in serial thin sections of fast-frozen and freeze- substituted high- and low-output active zones; and 4) the distance between the active zone particles and the active zone vesicles will be determined. These studies will be performed on muscles in which electrophysiological measurements have determined the transmitter release characteristics. These studies are important since human pathologies affecting transmitter output may cause deficits in central and peripheral nervous function. For example, Lambert- Eaton myasthenic syndrome, an autoimmune disease, reduces transmitter output at the neuromuscular synapse. Freeze-fracture studies have shown that this disease disrupts the ultrastructure of the active zone. Thus, understanding the relationship between active zone structure and the cellular mechanisms controlling transmitter release may lead to treatments of nervous diseases which disrupt synaptic function.